1. Field of the Invention
The present invention relates generally to solar-thermal power systems, and more particularly to novel and improved, lightweight and economical, reflective panels for use in such systems, together with methods of making the panels.
2. Prior Art
Solar heaters are known which include concave, trough-like reflectors for reflecting incident solar energy toward a fluid carrying conduit positioned above the reflective surface along its focal axis. A wide variety of uses have been proposed for such fluid heaters including distilling the heated fluid, extracting energy from the heated fluid in a turbine or in a heat exchanger, and storing the heated fluid for subsequent use. The referenced patents are typical of such proposed systems.
It has also been proposed to deploy large numbers of these solar heaters in a side-by-side array across such relatively arid landscapes as are typically found in the state of Arizona to provide vast terrestrial solar-thermal power systems. The function of such systems would be to generate electricity by passing the heated fluid through turbines.
While terrestrial solar-thermal energy plants have been proposed, the construction costs involved have prohibited their larger scale adoption. The cost of the reflective material alone is illustrative of this problem. In present day economics, a reflective surface is considered economical if it can be provided for a cost of about two dollars per square foot. This is the cost of the material which forms the reflecting surface itself, and does not include the required underlying supporting materials or the rotary mechanism whith movably carries the reflector to maintain its alignment with the sun.
As will be apparent, where many acres of land are to be covered with side-by-side arrays of these solar heater modules, the cost of the reflective material alone is quite high. This, coupled with the cost of the required support materials and their rotary mounts is prohibitive to the building of terrestrial solar-thermal energy plants that can compete favorably with the more conventional power generation systems in present day use.
In an effort to minimize the cost of the reflective material, construction economics would suggest that the parabolic reflectors be as wide as possible. Large reflectors spread over a designated ground area require somewhat less reflective material than do small reflectors since the small reflectors have more frequently spaced upwardly curved wall portions that must be covered with reflective material. Construction economics would likewise suggest the desirability of using the widest possible reflectors in order to minimize the number of collector modules required to cover a designated area, thereby minimizing the required number of upstanding rotary supports, the lineal feet of fluid conduit, etc.
Increasing the size of the reflectors is not, however, without its drawbacks. In reality, with present day reflector structures, there are a number of factors which come increasingly into play as reflector size is increased and which destroy all savings that would otherwise be gained by increasing reflector size. Some of these factors can be summarized as follows:
1. To begin with, the larger the reflector, the greater is the need for absolute accuracy of the parabolic reflective surface. Reflectors which are 10 or 12 feet in width and which focus incident solar radiation onto a 2 or 3 inch diameter conduit must not have a distorted reflective surface or the reflected radiation will bypass the conduit entirely. In short, as reflector size increases, so does its cost due to reduced tolerances requiring greater accuracy in the reflective surface contour.
2. Secondly, the larger the reflector, the greater is the need for its structural rigidity. It is not enough to accurately form a large reflector. For it to have any use, it must have sufficient inherent rigidity to retain the curvature of the reflective surface. In short, as reflector size increases, its thickness and weight and the complexity of its integral framework must also increase -- usually at a nearly exponential rate.
3. Thirdly, as reflector size and weight increase, so does the complexity and cost of the attendant structure for supporting and rotating the reflector to retain its alignment with the sun.
4. Fourthly, all this increase in size and weight and structural complexity vastly magnifies the problems attendant manufacturing, shipping, installing and servicing the collector modules.
In summary, while terrestrial solar-thermal power plants have been proposed, the problem of providing large inexpensive reflectors that do not require expensive supporting structures remains unsolved and stands as a barrier to the widespread acceptance of this essentially pollution-free approach to power generation.